DC arc furnaces having electrically conductive hearths are disclosed by the Stenkvist U.S. Pat. No. 4,228,314 and the Stenkvist and Rappinger U.S. Pat. No. 4,324,943.
The hearth of such a furnace is adapted to contain a melt of molten metal, the furnace having at least one arcing electrode with a tip positioned above the hearth and adapted to form a heating arc with the melt when the electrode and hearth are supplied with DC power.
In the patents, the hearth comprises a wear lining directly contacted by the melt and formed by refractory material through which electrical conductors extend from the bottom of the wear lining to the top for contact with the melt. Superimposed layers of electrically conductive bricks are laid beneath the wear lining and connect the bottom portions of the conductors with a metal conductor plate below and in electrical connection with the bottom layer of the electrically conductive bricks. The metal plate is adapted to be connected with the DC power circuit including the arcing electrode.
In U.S. Pat. No. 4,228,314 the refractory material through which the electrical conductors extend is formed by a refractory compound rammed on the conductive bricks with the conductors formed by metal rods extended through the compound. In U.S. Pat. No. 4,324,943 the refractory compound is formed by bricks through which the electrical conductors extend, or metal encased bricks may be used.
The electrical conductors used with either of the refractory materials, rammed compound or bricks, the electrically conductive bricks and the metal plate, are not only good electrical conductors as required for the arc's power circuit but are also correspondingly good thermal conductors. Consequently the heat of the melt heated by the arc is to a high degree transmitted to the metal plate. This can result in excessive heating of the metal plate and therefore the furnace bottom.
According to the Rappinger and Stenkvist application Ser. No. 601,348, filed Apr. 17, 1984, now U.S. Pat. No. 4,541,099, the layers of the electrically conductive bricks beneath the wear lining have an interposed layer in part of bricks of high electrical and thermal conductivity and in part of bricks of low electrical and therefore low thermal conductivity, the bricks being mixed and laid together to form the layer. The two kinds of bricks are laid to provide a spread distribution of the electrically conductive bricks interposed by the bricks of low conductivity. The distribution of bricks in the layer is such as to provide adequate electrical conductivity between the conductive brick layers but with the thermal conductivity substantially decreased by the presence of the bricks of low conductivity. This results in a substantial reduction in the temperature of the metal plate when the furnace is in operation.
This U.S. Pat. No. 4,541,099 further discloses a ladle furnace having DC arc heating with a hearth having a wear lining comprising a steel plate shell in the form of a cup having steel pins welded to it and extending inwardly with non-conductive refractory material rammed around the pins to form the wear lining as a unit. This lining can be made outside of the ladle and installed inside of the ladle during the ladle construction or repair. This wear lining unit rests on the electrically conductive brick layers with the layer of mixed bricks. With possibly other layers this hearth has the conductor plate beneath everything, the power being transmitted from the plate through the various conductive layers and steel plate shell to the hearth's electrically conductive wear lining.
In all of these previous proposals the hearth construction must be of undesirably vertical thickness to prevent overheating of the connector plate. This results in a furnace of undesirably large size.